Glyceryl monomethacrylate, hereinafter referred to in short as GMMA, can be obtained by hydrolysis of isopropylidene glyceryl methacrylate, hereinafter referred to in short as IPGMA.
GMMA has the following structure (A):
IPGMA has the following structure (B):
IPGMA is occasionally abbreviated in the literature to GMAK. Under common nomenclature, IPGMA can also be referred to as (2,2-dimethyl-1,3-dioxolan-4-yl)methyl methacrylate.
GB 852, 384 published in 1960 describes the production of GMMA by hydrolysis of IPGMA, the hydrolysis being carried out in the presence of dilute aqueous solutions of strong mineral acids. Sulfuric acid, hydrochloric acid, nitric acid and phosphoric acid are mentioned as examples of suitable mineral acids. The typical procedure is apparent from Example 1. According to this Example, the sulfuric acid present in the reaction mixture after the hydrolysis of IPGMA was removed by addition of barium hydroxide. Poorly soluble barium sulfate was formed and was removed by filtration. The crude target product GMMA was obtained as a ca. 20% solution in water/acetone (12:1). However, if it was intended to isolate the target product, the solution was saturated with NaCl, extracted with benzene or ether and the organic solvent removed.
Unfortunately, the process known from GB 852,384 has serious disadvantages. Thus, on account of its heavy metal content, the use of barium hydroxide involves a certain risk for ecological and toxicological reasons. The additional use of organic solvents is another unwanted factor. Benzene, which is both inflammable and toxic, is also used as solvent in Examples 2 to 4. It is clear that no neutralization takes place there and that the target product is directly extracted from the acidic aqueous reaction mixture with benzene. In addition to the use of benzene as solvent, this variant has the disadvantage that residual traces of acid in the target product GMMA cannot be ruled out. However, even traces of acid are extremely undesirable because they have a considerable bearing on the stability of GMMA in that they reduce its stability.
In addition, in their own studies based on reproducing Example 1 of GB 852,384, applicants observed the following: if the reaction mixture is neutralized with barium hydroxide and if the water is removed by stripping without extraction using an organic solvent, a GMMA with a high barium content (>1000 ppm) is obtained.
All in all, the method disclosed in GB 852,384 is unsatisfactory, particularly when a high-quality GMMA is required.
Even after twenty years from the GB patent cited above, there had still been no significant breakthrough in the production of GMMA. Thus, in column 4, lines 42 et seq of US-A-4,056,496 published in 1977, there is a reference to GB 852,384, more particularly to the hydrolysis and to the subsequent procedures disclosed therein. Accordingly, Example 1 of US-A-4,056,496 uses sulfuric acid for hydrolysis and barium hydroxide for neutralization, followed by filtration, addition of NaCI, extraction with benzene or ether and removal of the organic solvent.
And even another 20 years later, Example 1 of US-A-6,011,081 still refers to the “old methods”, i.e. to US-A-4,056,496.
High-purity GMMA may be used for various industrial applications, for example as a monomer for the production of polymers for coating purposes, lacquers, paints, adhesives or contact lenses. In the production of contact lenses in particular, GMMA is a technically very important monomer, cf. the art cited in EP-B-1,171,410 B1, page 2, line 6 to page 3, line 19.
The goal of providing high-purity monomers, such as GMMA, is very important above all because even trace impurities can have an extremely negative effect on the one hand during storage of the monomers and, on the other hand, on the quality of polymers produced therefrom. Particularly unwanted impurities in monomers are those which can act as crosslinkers in a polymerization reaction because the presence of crosslinkers during a polymerization reaction prevents and/or inhibits the formation of linear polymers.
Later patents EP-B-1 171 410 B1 and EP-B-1 171 411 B1 describe the production of GMMA by hydrolysis of IPGMA, a key feature of each patent being that the hydrolysis of IPGMA is carried out in the presence of an immobilized acid. According to Example 1 of EP-B-1 171 410 B1, IPGMA, deionized water and a cation exchanger are introduced into a reaction vessel and an air stream is passed 48 h through the mixture for the purpose of agitation. A disadvantage of the process described in these patents is that, through the passage of an air stream during the hydrolysis, the acidic ion exchanger undergoes serious erosion which is unacceptable where product purity has to meet stringent requirements and which cannot be completely removed even by subsequent filtration.
For the rest, several other disadvantages of the prior art represented by the PCT equivalents to EP-B-1 171 410 B1 and EP-B-1 171 411 B1 (namely WO 00/63149 and WO 00/63150) are mentioned in DE-A-103 49 972 (cf. page 2, paragraph [0007]and paragraph [0009]).
The technique of hydrolysis in the presence of an acidic ion exchanger is also described in later application DE-A-103 49 972 A1, although the process described therein does have to be carried out continuously. The reaction is carried out in a fixed bed and the carbonyl compound released by hydrolysis is continuously removed from the reaction medium.